The present invention relates to a method of and an apparatus for alleviating shift shocks occurring when an automatic transmission shifts between different transmission ratios.
Automatic transmissions are now widely used in automotive vehicles as a drive for driving an automotive vehicle. When the automatic transmission shifts from one ratio to another, an engine revolution rapidly changes due to a change in gear ratio, thus producing shift shocks mainly due to the inertia of the engine.
The automatic transmissions usually include a torque converter in an engine power delivery path thereof in order to multiply torque from an engine and absorb torque variations. The provision of the torque converter in the engine power delivery path causes a poor power transmission efficiency because of a slip in the torque converter. In order to alleviate this drawback, there has been proposed a torque converter with a so-called lock-up clutch which is is engaged to cause the lock-up action of the torque converter when the torque multiplication and torque variation absorbing functions are not needed. If a shifting operation is made with the torque converter in the lock-up state thereof, substantially great shocks take place because the torque variations upon shifting are not absorbed.
In the automatic transmission with such a lock-up torque converter, attempts to solve the above mentioned problem have been made by releasing the lock-up clutch to interrupt the lock-up action upon shifting.
Description is made regarding this in detail taking as an example a 2-3 shifting operation illustrated in FIGS. 7(a) and 7(b), which 2-3 shifting operation may be carried out by a known lock-up type automatic transmission disclosed in U.S. Pat. No. 4,431,095 issued to Masaaki SUGA on Feb. 14, 1984. According to this control strategy, upon lapse of a delay time T.sub.1 after an instant t.sub.1 when a shift command for 2-3 shift is made, the interruption of the lock-up action of the torque converter begins by turning OFF the lock-up signal at an instant t.sub.2 and then continues for a predetermined time T.sub.2.
However, an actual delay time from the instant t.sub.1 when the shift command is made to an instant t.sub.3 when the actual shifting operation begins varies from one manufacturing product to another, and thus the delay time T.sub.1 does not necessarily satisfy the optimal relationship with the actual delay, with the result substantial shocks cannot be alleviated to a sufficiently low level.
When the instant t.sub.2 is not timed well with the instant t.sub.3, the interruption of the lock-up action does not attain desired shock absorbing effect. That is, when the delay time T.sub.1 is too short and thus the instant t.sub.2 is advanced too much as compared to the instant t.sub.3 as illustrated in FIG. 7(a), the engine races to allow a rapid increase in engine revolution as against the variation in a transmission output shaft revolution (vehicle speed) which is moderate due to comparably great inertia of the vehicle body even after the instant t.sub.3. This causes the transmission output torque to vary with a substantially great peak T.sub.r1 which appears immediately before an instant t.sub.5 when the actual shifting operation is completed. This peak causes the generation of substantially great shift shocks. On the contrary, when the instant t.sub.2 is delayed too much as compared to the instant t.sub.3 , substantially great shocks are also produced because the interruption of the lock-up action is not yet initiated as the instant t.sub.3 when the actual shifting operation begins.
When the instant t.sub.2 is timed well with the instant t.sub.3 as illustrated in FIG. 7(b), the torque variation absorption function by the torque converter is fully utilized and the generation of the shift shocks can be alleviated to some degree. But there is the limit for the torque converter to absorb shift shocks, thus allowing a peak T.sub.r2 to take place which causes shift shocks.
A known attempt to reduce the shift shocks due to the peak T.sub.r2 is disclosed in U.S. Pat. No. 4,403,527. According to this known technique, the output torque of the engine is varied, such as by controlling at least one of the ignition timing, fuel supply and throttle opening degree, for a short period of time during the shifting operation with an intention to sufficiently alleviate shift shocks.